Alexander Pope warned that “a little learning is a dangerous thing.” Elizabeth Halsey (Cameron Diaz) has a little learning. Very, very little. She has less ambition. So the fact that she’s a middle school teacher makes her dangerous enough to be considered a WMD, incinerating the egos and minds of the fragile, hyper-hormonal pre-teens in her classroom. God forbid these kids get too close — high as she is, she might try to eat them for breakfast.

In theory, “Bad Teacher” should be a slam-dunk. With movie history littered with homages to dedicated, selfless teachers, who wouldn’t welcome a movie about an educator who hates teaching and sticks it to the education system every chance she gets? To a degree, that’s what director Jake Kasdan’s movie is, and it has the added bonus (for those who are, like, into that sort of thing) of a star who looks hot in platforms, jean cutoffs and a soaked plaid shirt. But while “Bad Teacher” has plenty of naughty lines, they’re all self-consciously naughty. They read like lines, and with Diaz’s delivery they feel completely artificial, hardly a natural extension of the character. It’s tough to buy into Elizabeth Halsey as anything other than a caricature — Jessica Rabbit, only blonde and with a pottymouth — because Diaz offers no nuance. She just looks bored.

The secondary characters in “Bad Teacher,” though, make things slightly more interesting. The best of the actors shine despite the lame gags (Justin Timberlake’s repulsive “wet jeans” scene comes to mind) and forced script. Phyllis Smith (“The Office”) supplies her trademark gawky humor and stellar comic timing as shy Lynn Davies, a fellow teacher and Elizabeth’s only friend. She warns Elizabeth about Amy Squirrell (Lucy Punch, highly entertaining), the comically malicious busybody who romances the rich new sub, Scott Delacorte (Timberlake), before Elizabeth can get her money-grubbing hooks into him. Punch, who demonstrates a lovely lack of vanity, goes all-out to earn every laugh, and Amy’s unbridled desperation to win at everything only adds to the comedy. Jason Segel’s Russell, the average-guy gym teacher Elizabeth spurns repeatedly, has a few genuinely amusing moments, addressing one of his pale, artfully scruffy-haired students as “Twilight” and vehemently arguing with another that LeBron James is no Michael Jordan. The misfire (and it’s a sad one) is John Michael Higgins, comedian extraordinaire whose role as dolphin-crazy Principal Wally Snur is far too small. Given room to run, Higgins could have lived up to his character’s odd and inexplicably funny last name.

Least interesting of all these is Elizabeth, who’s despicable up one side and down the other: rude, self-absorbed, petty, obsessed with money, possessed of a nasty sense of entitlement. She thinks the world owes her a living. These kinds of parts can be dynamite comedy with the right actors (free shots to Billy Bob Thornton’s Willie T. Soke). Kasdan, however, seems to think audiences will find nastiness endearing because it’s Cameron Diaz in sky-high heels who’s being naughty. How misguided he is. Bad behavior is fun, occasionally even affecting, when it serves a purpose. In James Mottern’s “Trucker,” for example, Michelle Monaghan’s rough-at-the-edges charm made for an unpredictable mother/son story. Here, Diaz succeeds in the broad physical comedy (think “The Sweetest Thing”) but lacks the nuance to pull of the Elizabeth. She can’t manage to give depth to the character. And the appeal of a hot, bored woman smoking a bowl in her car in the school parking lot, slamming her students in the face with kickballs and dry-humping a coworker’s boyfriend is decidedly limited. Diaz has made a profitable career of coasting on her hotness. That doesn’t mean Kasdan should too. It’s a lazy choice, and it derails “Bad Teacher” way before it can rumble and squeak its way to a pitiful, completely illogical ending.

Jason Segel has a face made for break-up movies. Or just break-ups, period. Whether he’s warbling a serenade for the woman of his dreams (the notorious “Lady” scene in “Freaks and Geeks”) or crying naked in front of his just-became-ex-girlfriend, there’s a congenial openness to Segel’s face that is appealing. He may be an actor, but he looks like the down-to-earth sort who would wear Costco sweatpants, eat giant bowls of Fruit Loops in front of the TV and drink grocery store wine. This is a big reason why Segel’s labor of love and humor, “Forgetting Sarah Marshall,” is so enjoyable: it’s funny and perceptive without being pretentious, and it’s endearing but not mushy or overly sentimental. “Forgetting Sarah Marshall” is a realistic romantic comedy unafraid to let everything hang out … figuratively and literally.

Segel’s male perspective also gives the genre a welcome and refreshing twist. While so many rom-coms sing the “good woman done wrong” blues, “Forgetting Sarah Marshall” offers a different tune. This time around it’s the nice guy who’s had his heart turned into a smooshed MoonPie. Peter (Segel) loves the blonde, petite and beautiful Sarah Marshall (Kristen Bell, terrific), a high-profile actress. But there’s a problem: Sarah’s career has turned her life busy and exciting, while Peter is at a dead standstill. When Sarah, frustrated with his homebody attitude, dumps him (in the best break-up scene ever written), Peter’s whole world collapses. He turns wallowing into an art form. Finally, a miserable and slovenly Peter takes the advice of his stepbrother (Bill Hader) and flies off to Hawaii for a break. Enter Life Interruption No. 2: Peter ends up at the same hotel as Sarah … who is there with her new boyfriend, Aldous Snow (Russell Brand) … who is a rich, famous rock star and bonafide sex god in leather pants.

From this point on, Segel puts his own flourish on the romantic comedy formula, providing minor tweaks here and there and adding in a host of comical, unusual, even touching secondary characters. Peter does meet a girl, hotel concierge Rachel (Mila Kunis), but she is not a damsel waiting to be whisked away from her unhappy life. She’s also the antithesis of Sarah Marshall’s spoiled, self-absorbed diva-in-training: Rachel is funny, kind and content with her life. She coaxes Peter out of his drunken, weepy stupor, encourages him to take a few risks, pursue his odd dream — write a puppet rock opera about Dracula — and get on with his life. Kudos to Segel for writing a potential love interest who is no selfless savior type. He deserves some high-fives, too, for crafting minor characters who are as funny as they are interesting. Anxious newlywed Darald (Jack McBrayer) worries himself sick about his lack of sexual prowess. Paul Rudd plays against his usual hyper-sarcastic type as Chuck, a perpetually fried and apathetic surfing instructor who lives by his own slacker credo: “When life gives you lemons, just say ‘fuck the lemons’ and bail.” That’s fortune cookie wisdom at its most original.

The real standout, and the clearest indicator that Segel wants to do things his own way, is Aldous Snow. In a less imaginative film, Aldous would be a sneering, six-packed villain of the vilest order, or a brainless moron to be ordered about; in “Forgetting Sarah Marshall,” he’s friendly, witty, charming and often quite insightful. Brand delivers the rocker’s many insights as only Russell Brand can: with a mix of bravado and cheek. He compares vacationing with the demanding Sarah to going on holiday with Joseph Goebbels, and when creepy fan Matthew (Jonah Hill) asks him if he’s listened to his demo, Brand’s retort is killer: “I was gonna listen to that, but then, um, I just carried on living my life.” In fact, Aldous — who later got his own movie, “Get Him to the Greek” — may be the most layered character in the film. Anyone who complains about the small female roles missed the point. “Forgetting Sarah Marshall” isn’t about women, much the same way “Sex and the City” wasn’t about men. Segel simply means to tell a personal and painful story from a male perspective, and he does — flaccid penis and all.

Can you say “dream cast”? Universal’s “Despicable Me” has one, alright, and it includes everyone from Jason Segel (Nick Andopolis!) to Julie Andrews to Russell Brand (the guy whose hair looks like mental illness!). Oh, and silly me — I forgot Steve Carell. Catch a glimpse of the trailer and join me in the long, long wait until July 9, 2010.

Confession Number Three: When I heard the twain would meet in “I Love You, Man,” I began grasping for the smelling salts to fend off the fainting spell I knew was coming. Then I hunkered down and waited for March 20.

The point of this little closet unloading session? To proffer a warning: What you’re about to read is a review replete with bias. (You’ll probably walk away with a little on your shoe.) Sure, I could try to push it aside, but why? Truth be told, Rudd and Segel are so good together — think Felix and Oscar meet Ren and Stimpy — they deserve heaps-o-praise … even if it is delivered by someone who once made a Paul Rudd collage (I feel your scorn, and I accept it).

But first we must wade through the pat setup: Peter’s more Merlot than Budweiser, so he hits a wall when his fiancee Zooey (Rashida Jones, trying to live down that undeserved “Is that Karen from ‘The Office’?” rep) asks him his pick for best man at their wedding. Oops. Peter spent so much time being the “girlfriend guy” he never made male friends. His family — including Jane Curtin as mom and scene-stealer Andy Samberg as his brother Robbie — set him up on a string of man dates with uniformly disastrous results. (Watch for the hilarious cameo by “Reno 911!” alum Thomas Lennon.) Overall, been there, seen that. Yawn.

The real fun starts when Sydney shows up at an open house Peter’s hosting to sell the palatial estate owned by Lou “The Hulk” Ferrigno. Sydney wolfs down the paninis, scouts the scene for divorcees, upbraids a farting interloper trying to impress an out-of-his-league date … and Peter falls hard. Like any couple relishing new love, they can’t get enough of each other: strolls down Venice Beach, Rush concerts, impromptu jam sessions in Sydney’s ill-lit man cave (it has a widescreen plasma! and a bitchin’ drum set!).

And so the story goes. The plot has all that you’d expect of a rom-com: boy meets boy, boy falls for boy … you get the picture. Even the pat ending feels like something that would fit fine in, say, “Sweet Home Alabama II.” Ignore all that. The real meat’s in the performances, and “I Love You, Man” is packed with great ones. The supporting cast is bang-on, including Jaime Pressley and Jon Favreau as a hilarious bicker-happy married couple and relative newcomer Sarah Burns, who comes off like a younger, bouncier Kristen Wiig. J.K. Simmons shows up as Daddy Claven to do what he does best: fire off dependably witty one-liners. Samberg, whose biggest role thus far has been Rod Kimble in “Hot Rod,” is droll perfection as a gay fitness instructor who’s grown bored with pursuing gay men and has set his sights on tougher game: the average married straight man. Yeah, he’s funny, but more impressive is his subtlety. It sneaks up on you.

And a moment of meditation on Rashida Jones: There’s a reason she was chosen for this part. She’s got the comedic timing and the pluck needed to make Zooey much more than a stereotypical needy, nagging fiancee. Here’s to hoping Hollywood wises up to her considerable talents before she’s lost to TV world forever.

But this is Rudd and Segel’s show, and they do not disappoint. Both are gifted comedic actors skilled at revealing vulnerability and humanity through comedy. They know it’s the details that matter. For Rudd, it’s all about painful pauses and inappropriate reactions. In Guy World, Peter’s the exchange student who doesn’t speak the language, doesn’t know the customs but thinks he can fake it. He can’t, but watching him try is priceless. (Note with glee the various awful nicknames he dreams up for Sydney, including “Jobin” and “Totes Magotes.”) There’s something endearing about his complete ineptitude. Sydney, on the flip side, is all confidence and wild-eyed spontaneity — an unusual Tony Robbins/Gallagher hybrid. True, he’s a loud-mouthed oaf with a bad case of arrested development, but Segel shows Sydney’s smarts, kindness and also the fear lurking beneath his macho posturing. Segel and Rudd are a match made in heaven (I won’t say I told you so … I won’t say I told you so).

And that’s the thing about “I Love You, Man”: It touches on the truth that finding a best friend is a lot like falling in love. Man, woman — it doesn’t matter. Except if it’s men, there’s the slight chance a man cave with a “love your member as yourself” corner will be involved.